1. Field of the Invention
The disclosed technology relates to optical shear sensors, more in particular to compact, flexible and/or stretchable optical shear sensors and to methods for fabricating such shear sensors.
2. Description of the Related Technology
For several applications, such as for example for skin friction measurements (e.g., diabetic patients, shear between prosthesis and skin) and for robotic applications (artificial skin) there is a need for compact shear sensors and shear sensor arrays allowing distributed shear sensing (tactile sensors). Preferably such shear sensors and shear sensor arrays are flexible and/or stretchable such that they can be adapted to conform to 3D surfaces. It is an additional advantage of flexible and stretchable sensors or sensor arrays that they may have a better reliability in terms of mechanical stability (e.g. shock and vibration absorption) as compared to rigid sensors and sensor arrays.
There have been several reports on tactile sensors that can measure shear stresses. Most of these sensors are fabricated using silicon micro-electro-mechanical systems (MEMS) technology. It is an advantage of using MEMS technology that it allows implementing tactile sensors with a fine spatial resolution and sensitivity comparable to human fingers. One of the disadvantages related to MEMS-based sensors is that they are not flexible because they are formed on a rigid silicon substrate. MEMS-based sensors often comprise floating elements, which may limit their robustness.
Shear sensors can be based on electrical measurements, such as for example capacitive measurements, magnetoresistive measurements or piezoelectric measurements. It is a disadvantage of sensors based on electrical measurements that they may be affected by electromagnetic field interference and that they may suffer from hysteresis and drift. Therefore, optical sensors may be preferred. Apart from their immunity to electromagnetic field interference, optical sensors can have a high sensitivity and a large dynamic range, and a highly linear response. Optical sensors are potentially compact and they can be embedded in or attached to an object to be sensed.
Optical shear sensors based on fiber-optic bend loss have been proposed. The basic configuration of such a fiber-optic shear sensor is a multi-layered sensor in which the top and bottom layers are composed of a pressure sensor mesh, each mesh comprising two sets of parallel fiber planes. Coordinates of pressure points are taken from the top and bottom mesh sensors to determine shear. Pressure points that are originally located above each other are shifted out of alignment because of shearing forces, and the amount of misalignment is related to the amount of shear. However, the spatial resolution that can be obtained by this type of shear sensor is limited. For example, in “A shear and plantar pressure sensor based on fiber-optic bend loss”, W-C. Wang et al, Journal of Rehabilitation Research & Development, Vol. 42, No. 3, pages 315-326, May/June 2005, a spatial resolution of 1 cm is reported.
In U.S. Pat. No. 7,295,724 a flexible optical distributed shear sensor is described, the sensor comprising a flexible substrate with a waveguide formed thereon. The waveguide comprises several Bragg gratings along the waveguide path, each Bragg grating having a characteristic Bragg wavelength that shifts in response to an applied load due to elongation or compression of the grating. These wavelength shifts are monitored to determine the amount of applied pressure on the gratings. To measure shear stress, two flexible substrates with the waveguide and Bragg gratings are provided on top of each other such that the waveguides and gratings are perpendicular to each other. Read-out of this type of sensors is rather complicated and expensive, as it requires spectral measurements.
In U.S. Pat. No. 7,466,879 a flexible optical shear sensor is described wherein an optical fiber with a Bragg grating is embedded at a small angle within a deformable layer, between an upper layer and a lower layer, a first part of the optical fiber being anchored to the upper layer and a second part of the optical fiber being anchored to the lower layer. When a shear force is applied on the upper surface of the sensor, the upper layer moves with respect to the lower layer in the direction of the applied shear force. This relative movement between the upper layer and the lower layer deforms the rubber matrix and leads to stretching of the embedded fiber, resulting in a shift of the Bragg wavelength. Read-out of this type of sensors is rather complicated and expensive, as it requires spectral measurements.